Size reduction of metal-oxide-semiconductor field-effect transistors (MOSFETs), including reduction of the gate length and gate oxide thickness, has enabled the continued improvement in speed, performance, density, and cost per unit function of integrated circuits over the past few decades. To further enhance transistor performance, MOSFET devices have been fabricated using strained channel regions located in portions of a semiconductor substrate. Strained channel regions allow enhanced carrier mobility to be realized, thereby resulting in increased performance when used for n-channel (NMOSFET) or for p-channel (PMOSFET) devices. Generally, it is desirable to induce a tensile strain in the n-channel of an NMOSFET transistor in the source-to-drain direction to increase electron mobility and to induce a compressive strain in the p-channel of a PMOSFET transistor in the source-to-drain direction to increase hole mobility. There are several existing approaches of introducing strain in the transistor channel region.
In one approach, strain in the channel region is introduced by creating a recess in the substrate in the source/drain regions. For example, a PMOS device having a compressive stress in the channel region may be formed on a silicon substrate by epitaxially growing a stress-inducing layer having a larger lattice structure than the silicon, such as a layer of SiGe, within recessed regions in the source/drain regions. Similarly, an NMOS device having a tensile stress in the channel region may be formed on a silicon substrate by epitaxially growing a stress-inducing layer having a smaller lattice structure than the silicon, such as a layer of SiC, within recessed regions in the source/drain regions.
In this approach, the stress-inducing layer is limited to the more heavily-doped regions of the source/drain regions. As a result, there is little or no contribution to stress in the channel region from the source/drain extension regions or the overlapping region between the gate electrode and the source/drain extensions.